Technical Field
The present disclosure relates to transmit/receive channels for ultrasound applications.
Description of Related Art
An ultrasound imaging or ultrasonography machine is known to be a medical diagnostic testing system that uses ultrasonic waves or ultrasounds and is based on the principle of ultrasound transmission and echo emission analysis and is widely used in internal medicine, surgery and radiology.
Typically used ultrasounds range from 2 to 20 MHz.
Frequency is selected considering that higher frequencies have a greater image resolving power, but penetrate to a shallower depth in the individual under examination.
These ultrasounds are typically generated by a piezoceramic crystal in a probe that is kept in direct contact with the skin of the individual with the interposition of an appropriate gel (which is adapted to eliminate air between the probe and the skin of the individual, thereby allowing ultrasounds to penetrate the anatomic region being examined).
The probe can collect a return signal, or echo, which is appropriately processed by a computer and displayed on a monitor. Particularly, ultrasounds that reach an acoustic impedance variation point, such as an internal organ, are partially reflected and the percentage reflection provides information about impedance differences between the penetrated tissues.
The time that an ultrasonic wave takes to run its path of propagation, reflection and return is provided to the computer, which calculates the depth from which the echo is emitted, and thus identifies the boundary surface between the penetrated tissues (which corresponds to the acoustic impedance variation point and hence to the depth from which the echo is emitted).
A typical transmit/receive or TX/RX channel that is used in these applications is schematically shown in FIG. 1 and generally designated by numeral 1.
Particularly, the transmit/receive channel 1 comprises a high voltage multi-level shifter 2 of the type comprising a branch 3 inserted between a first terminal HVP connected to a positive voltage, and a second terminal HVM, connected to a negative voltage.
In the illustrated example, also referring to FIG. 1A, the shifter 2 allows switching between at least two levels, e.g., the high level, corresponding to the voltage of the first terminal HVP and the low level HVM.
Typical values for the terminal HVP connected to a positive voltage range from 5V to 100V, whereas typical values for the terminal HVM connected to a negative voltage range from −5V to −100V.
The voltage of the output terminal HVout of the level shifter 2 is clamped by a clamping block 4 to a reference voltage, in this example the ground voltage GND, whereby an additional level to be reached by the shifter 2, e.g., GND, is introduced, as shown in FIG. 1A.
It shall be noted that, in a common waveform as shown in such FIG. 1A:
the positive edge is the transition from the HVM value to the HVP value,
the negative edge is the transition from the HVP value to the HVM value,
the positive clamp is the transition from the HVM value to the GND value and
the negative clamp is the transition from the HVP value to the GMD value.
The clamping block 4 is substantially a high-voltage switch inserted between said output terminal HVout of the level translator 2 and the ground voltage GND.
Particularly, the clamping block 4 comprises a first driver DRC1, which is designed to control the gate terminal of a first clamping transistor P2, which is in turn inserted in series with a first clamping diode DC1, between the voltage reference GND, particularly a ground, and a central node XCc.
The clamping block 4 comprises a second driver DRC2, which is designed to control the gate terminal of a second clamping transistor N3, which is in turn inserted in series with a second clamping diode DC2, between the voltage reference GND, particularly a ground, and a central node XCc.
The central clamping node XCc is further interconnected with the output terminal HVout of the transmission channel 1, which is in turn connected to a connection terminal Xdcr for the piezoelectric transducer 5 to be controlled by the transmission channel 1.
More in detail, the high-voltage shifter 2 comprises a first branch, having a first switching transistor P1 and a second switching transistor N2, which are inserted in series with each other between the first higher voltage reference terminal HVP and the first lower voltage reference terminal HVM. The first and second transistors P1 and N2 have respective control terminals connected to and controlled by first DRP1 and second DRN2 input drivers, and the respective drain terminals connected together.
It shall be noted that the first switching transistor P1 is a high-voltage P-channel MOS transistor (HV Pmos), whereas the second switching transistor N2 is a high-voltage N-channel MOS transistor (HV Nmos), whereas the first and second transistors P2, N3 of the clamping block 4 are always high-voltage transistors, of P-MOS and N-MOS types respectively, but can withstand a lower supply voltage (e.g., 100 V against the 200 V of the first and second switching transistors P1, N2).
Therefore, in classical ultrasonic solutions, the level shifter is obtained using asymmetric output stages (NMOS and PMOS) which generate a second harmonic distortion.
Thus, when the transmit channel 1 is required to be switched from a high-voltage level (e.g., HVP) to a low-voltage level (e.g., HVM) or vice versa, through the positive and/or negative clamping steps, the shifter 2, which is constructed with MOS transistors of different types (Nmos vs Pmos), with the clamping block being constructed with transistors of different classes, has different transitions to the output terminal HVout depending on whether the shifter is switched to a high-voltage value or a low-voltage value.
This asymmetry in the rising or falling edge of the voltage signal to the terminal HVout causes a second harmonic component to be introduced into the emitted signal, and thus disturb later second-harmonic analysis on the reflected echo.
Generally, this asymmetry may be minimized according to the current/voltage characteristics of the two Nmos and Pmos transistors, by having them operate in appropriate range of operating conditions (such as output load and operating voltage).
Nevertheless, this optimization is not stable and accurate and especially, with changing operating conditions, it may lead to a considerable degradation of performances, possibly to 10 db lower attenuation of the transmitted second harmonic component.
This introduced asymmetry particularly affects the percentage of the reflected acoustic signal, which carries information about the difference in impedance between the penetrated tissues.
This second harmonic distortion is tolerated (in non-high-quality applications), when it is attenuated by about 30 db to 40 db with respect to the value of the carrier of the generated acoustic signal.
Nevertheless, due to this distortion, the images of the region to be observed are generated with a resolution that is lower than the one that might be obtained without such asymmetry.